In recent years, development within offshore oil and gas exploration has been directed to subsea installations for processing and transport of oil and gas. Such subsea installations replace the traditional platforms, where oil and gas was transported up to the platform for further processing and transport. This development of subsea production, processing and transport systems has resulted in an increasing need for subsea connection arrangements for coupling together a pipeline being laid from a surface vessel with a static structure installed on the seabed. The pipeline may subsequently be connected to another pipe termination, such as for example a subsea production or injection well, possibly using a so-called spool piece without in most cases requiring any assistance of divers. The spool piece is designed to establish a flow connection between the pipeline end structure and the pipe termination. The spool piece is provided with a spool piece hub which is to mate with a corresponding pipeline hub arranged at the pipeline end structure.
The process of connecting the pipeline to the static subsea structure is not straightforward. Moreover, the process may differ depending on the water depth. The end of the pipeline is provided with an end structure, which will typically be a pipeline end termination (PLET), pipeline end module (PLEM), flexible line or flowline end termination (FLET) or skid.
In shallow water, the process typically comprises the following steps:                (i) An anchor, generally a drag-type anchor, is first installed on the seabed close to the desired target area.        (ii) An initiation cable, usually wire rope, is then selected to suit the anticipated initiation loading, taking account of a suitable safety factor. The initiation cable is made of length such that the end structure can reach its target area.        (iii) The initiation cable is then recovered from the sealed/anchor or deployed from a winch on the surface vessel and is connected to the anchor and to the end structure while it is still on the surface vessel.        (iv) The surface vessel is then moved to a predetermined distance from the seabed anchor until the initiation cable is held at a tension determined by the specific installation parameters.        (v) The pipeline is then deployed from the surface vessel in accordance with job-specific installation parameters. As the pipeline is deployed, the lay equipment angle is adjusted to adjust the pipe departure angle, and the vessel is moved in the direction of pipe-lay, whilst maintaining suitable tension in the initiation cable and the pipeline. This process is continued until the end structure lands on the seabed, after which normal pipe-lay can commence.        
In deep water, the process is typically as follows:                (i) An anchoring device, which may be a drag anchor, a suction pile or a suction can, is installed at a predetermined distance from the target area for the end structure. In most cases, this distance will be in excess of 15 to 25 meters, and is determined by the dynamic loads and stresses expected to be imposed on the pipeline during the installation process.        (ii) An initiation cable, typically a wire rope, is sized to suit the expected initiation loading, taking account of a safety factor. The initiation cable may consist of two parts: one part pre-connected to the anchoring device and another part connected to the end structure. The length of the cable is selected such that, once the anchoring device is connected to the end structure, the end structure can be landed on the predetermined target area. The target area is sized to accommodate design tolerances for the subsequent tie-in of the end structure to another structure or well head, using a rigid pipe spool, rigid jumper or flexible jumper. It will be understood that as the distance between the anchoring device and the target area becomes shorter, the dynamic loads and stresses in the pipeline become higher as a result of the environmental conditions, determined by a combination of vessel characteristics, wave height, wave period, wave direction, current profile and current direction. If a relatively short initiation cable is used, stricter weather criteria may need to be set to allow installation to take place without over-stressing the pipeline.        (iii) The pipeline with its end structure is deployed vertically from the surface installation vessel until the initiation cable reaches a position at which it can be connected to the anchoring device or anchor cable by means of an ROV (remotely operated vehicle).        (iv) The surface vessel is then moved to a predetermined distance from the seabed anchoring device such that the initiation cable is held in tension, according to job-specific installation parameters.        (v) Laying of the pipeline is then initiated step-by-step in accordance with job-specific installation parameters. As pipe is deployed from the vessel, the lay equipment angle is modified to adjust the pipe departure angle, and the vessel is moved in the direction of pipe laying, whilst maintaining tension in the initiation cable. These steps are continued until the end structure lands on the seabed and normal pipe-lay can commence.        
One pipeline connection arrangement and method for subsea connection of a pipeline to a spool piece hub is described in U.S. Pat. No. 4,019,334 A. The spool piece hub is here attached to a foundation arranged on the seabed. The pipeline end structure, including a pipeline hub, is secured to a trunnion assembly and is installed on the subsea foundation by being lowered together with a trunnion assembly downwards from a surface vessel guided by a guide cable. After the pipeline hub and the trunnion assembly have been landed on the subsea foundation and secured thereto, a remotely operated connecting tool is lowered downwards from the surface vessel and is operated to displace the spool piece hub into engagement with the pipeline hub, whereupon the connecting tool is operated to actuate a clamping device so as to clamp together the spool piece hub and the pipeline hub. The connecting tool is then retrieved to the surface vessel. This known connection arrangement and method require that the spool piece and the subsea foundation are installed on the seabed before the pipeline is laid out on the seabed and the pipeline end structure has to be secured to the subsea foundation before the remaining part of the pipeline is laid out on the seabed.
Another pipeline connection arrangement for subsea connection of a pipeline to a spool piece is known from U.S. Pat. No. 6,142,708 A. The pipeline hub is here secured to a landing platform which is to be lowered onto the seabed from a surface vessel. The spool piece hub is secured to a connecting tool which is lowered downwards onto the landing platform so as to come to rest thereon, whereupon the connecting tool is operated to displace the spool piece hub into engagement with the pipeline hub. The pipeline hub is secured to the rather complex and bulky landing platform before being lowered into the sea from a surface vessel. The lay ramp and other equipment of the pipe laying vessel have to be adapted and dimensioned for enabling manipulation and lay-out of the landing platform. This raises the cost for the pipe laying vessel and reduces the number of vessels available for the pipe laying operation which could cause high costs for performing the pipe laying operation.
US Patent Application No. 2007/0269270 A1 describes a method of enabling subsea connection of a pipeline to a spool piece without requiring any complex and/or bulky equipment to be attached to the pipeline end structure before it is lowered into the sea from a surface vessel. In this method, a pipeline termination skid is so designed that a pipeline end structure may be mounted thereto subsea by lowering the pipeline end structure into a seat at one or several holding devices of the pipeline termination skid and thereafter displacing the pipeline termination skid along the pipeline end structure so that the respective seat is brought into engagement with an alignment element secured about the pipeline end structure so as to thereby fix the pipeline end structure axially and radially in relation to the pipeline termination skid.
Another system for interconnecting two pipelines at a subsea position is described in U.S. Pat. No. 5,807,027 A. A first pipeline is initially freely suspended whilst a second pipeline is mounted on a bottom base to a manifold frame. The end section of the first pipeline is provided with a socket type termination with a front end which is provided with means for coupling of the terminator to complementary pipe coupling means on the second pipeline. The terminator is provided with a laterally directed longitudinally shaped anchor element and the manifold frame is provided with receiving means for receipt and fixation of the anchor element. The anchor element and the receiving means are dimensioned and positioned such that when the anchor element is placed in position in the receiving means, the coupling means of the terminator will be positioned directly in front of the complementary coupling means on the second pipeline.
WO 2008/063080 teaches various approaches for achieving a guide and hinge-over connection between the end of a first pipeline being laid and the end of a second pipeline already installed on the sea floor. In one of these embodiments, a connector at the end of the first pipeline is provided with a bail structure consisting of two arms linked by a bar. The bar is received in a frame on a connector of the second pipeline. When the bar is located in the slot, the connector on the first pipe pivots round until the pipe ends are in position for engagement with each other.
It is also possible to lay pipeline with a pipeline end termination (PLET) or manifold (PLEM) attached to the leading end. However, these are large devices typically weighing between ten and fifty metric tonnes. When such a large and heavy device is lowered on the end of the pipe from a surface pipe lay vessel, difficulties may arise where the surface vessel is subject to heave which may be plus or minus one meter or more. It is difficult to land the PLET or PLEM safely without damage at a known location and orientation under conditions of vessel heave which may be transmitted to it down the pipeline.
An object of the invention is to enable capture of a PLET/PLEM in a device on the seabed without causing damage to the subsea structure or overstressing the pipeline as a result of the effect of vessel heave.
Another object of the invention is to enable initiation of a pipeline in a congested seabed area where conventional techniques using anchors and winch cables may not be feasible.